1. Field of the Invention
The present invention relates generally to transmission controls, and more particularly to an inertial phase fluid pressure control apparatus and method for an automatic transmission.
2. Description of Related Art
Generally, a vehicle automatic transmission includes both a torque converter and a speed-changing gear mechanism. The transmission automatically changes among predetermined gears via a speed-changing gear mechanism power transmission path that is switched by selective operation of a number of friction elements such as clutches and brakes.
Such an automatic transmission includes a hydraulic pressure controller for controlling the charge and discharge of working fluid to and from the frictional elements. During gear shifting operations, a smooth transition is realized through precise control of the charge and discharge of working fluid by the hydraulic pressure controller.
For example, to reduce shift shock in the inertia phase of the gear-changing transition, feedback control is performed to cause the input shaft speed of the speed-changing gear mechanism to trace a target value during the inertia phase period. To reduce inertia torque, which is caused by shift shock, torque-reducing control of the engine is executed.
Because it is preferable for such feedback control and torque-reducing control to be executed from the start of the inertia phase, the detection of the starting time of the inertia phase is necessary when these controls are to be executed.
For this detection of the start of the inertia phase, a method is used that takes into account any acceleration of the vehicle, using the input shaft speed Nt, the output shaft speed No and the gear ratio before the gear change gr, by calculating F from the following Exp. (1), and when F.gtoreq.0 this is taken as the start of the inertia phase. EQU No.multidot.gr-Nt=F (1)
More particularly, considering variation of speed signal detection values and the influence of noise, a method is utilized wherein the start of the inertia phase occurs when F.gtoreq..DELTA.n (.DELTA.n being a predetermined determination value), that is, when the following Exp. (2) is satisfied. EQU No.multidot.gr-Nt.gtoreq..DELTA.n (2)
For example, Japanese Unexamined Patent Publication No. H.1-266025 relates to a technology wherein the start of the inertia phase is detected by the above method and torque-reducing control of the engine is started on establishment of the condition in Exp. (2).
According to Japanese Unexamined Patent Publication No. H.2-80853, on the other hand, starting feedback control immediately after the establishment of Exp. (2) is undesirable from the viewpoint of the rotation behavior of the input shaft. It is preferable for feedback control to be started after a delay of a predetermined time, or after the gradient of the decrease of the input shaft speed reaches a predetermined value. In Japanese Unexamined Patent Publication No. H.8-270780, this determination value of the decrease gradient of the input shaft speed is set according to the size of the input torque at the time of the gear change.
However, in carrying out research into automatic transmission control, with the kind of method described above, the present inventors have encountered conditions such that the start of the inertia phase, i.e. the starting point for feedback control and torque-reducing control, cannot be reliably detected.
That is, as shown in FIG. 17, before the start of the inertia phase, there is a phenomenon of the input shaft speed Nt bounding. By carrying out research into this phenomenon it was ascertained that the phenomenon of the input shaft speed Nt bounding occurs under the following two conditions (with the following causes) [1] and [2].
[1] Referring to FIGS. 17A and 17B, the first condition is a result of coefficient of friction characteristics of friction elements.
As shown in FIG. 17B, the friction element generally has the property that its coefficient of friction temporarily assumes a large value when the clutch initially engages. Because of this, in the initial stage of shifting, the frictional force temporarily assumes a large value. Consequently, the gear change proceeds and the input shaft speed Nt temporarily starts to fall (a fall in the torque phase) as shown in FIG. 17A. However, because at this time the hydraulic pressure is still not sufficiently high, when the coefficient of friction subsequently decreases to its essential level, it cannot maintain an engagement force sufficient to continue the gear change, and the input shaft speed Nt rises again due to torque applied by the engine. After that, as the clutch hydraulic pressure rises, the real gear change begins. As a result, the input shaft speed Nt truly falls.
[2] Referring to FIG. 17C, the second condition occurs in gear changes in vehicles with one-way clutches, when the accelerator is depressed but then minimally returned so that a shifting line is crossed.
At this time, as shown in FIG. 17C, first, as a result of an engine braking effect caused by the accelerator being returned, the input shaft speed Nt temporarily falls. Then, the one-way clutch ceases to transmit the load torque from the vehicle side (the output shaft torque). As a result, the inertia acting on the input shaft falls. Consequently, the falling of the input shaft speed Nt stops or the input shaft speed Nt rises slightly. Thereafter, progress of the real shifting causes the input shaft speed Nt to fall.
When a bound occurs in the input shaft speed Nt due to the cause described above, the start of the inertia phase is determined on the basis of Exp. (2). For example, as illustrated in FIG. 17A, at the first fall in the input shaft speed Nt (see "A" in the figure), it is mistakenly determined that the inertia phase has started.
Consequently, when feedback control and torque-reducing control are initiated, because the state of the engaging (or oncoming) clutch is still at the end of the torque phase, and shifting has not proceeded to the inertia phase, the movement of the input shaft speed Nt cannot be properly adjusted by hydraulic pressure control. Also, because an inertia torque has not developed, when torque-reducing control is executed, a large shift shock occurs.
As a method of preventing the above type of erroneous detection, the determination value (.DELTA.n) of the above-mentioned Exp. (2) for detecting the inertia phase may be set to a sufficiently large value. However, with this method the detection of the starting time of the inertia phase is delayed, and the starting point of inertia phase control is therefore delayed. Consequently, it is not possible to realize a sufficient reduction in shift shock.
Also, when the method of detecting the decrease gradient of the input shaft speed Nt is utilized, because the affect of acceleration of the vehicle is not reflected in the detection, the detection of the start point of the inertia phase is also delayed.